The past decade has seen a revolution in computing. The advent and proliferation of personal computers has transformed the computing environment from one which was highly centralized and tightly controlled to one which is widely distributed with easy access. A significant expansion of computing applications is a concomitant result of this changing computer environment. In the past, computers provided, primarily, accounting, data reduction, and data-base management functions. They now, additionally, provide voice messaging, games, and multimedia applications, such as business presentations. While the older applications could be accommodated using only text-type data, the newer applications require graphical and audio data as well.
Graphical and, more to the point of this invention, audio signals require significant capacity for storage. For example, the word "hand" would require 4 bytes, 1 byte for each letter, for storage as text. On the other hand, the storage required for a digital audio version of "hand", assuming pulse code modulation (PCM) with 16 bits per sample with 20,000 samples per second, and assuming 1 second is required to utter "hand", is about 40,000 bytes. Although the cost-per-bit of computer storage has fallen dramatically, limited storage still imposes severe constraints on computer applications which use digitized audio. Consequently, it is highly desirable to compress digitized audio signals for use in multimedia computer environments.
Audio signals are commonly digitized using pulse code modulation (PCM) techniques. Pulse code modulation is applied by sampling an analog audio signal at a fixed rate, for example, 20 kHz, to produce a stream of pulse samples. The modulation technique then assigns a digital value to each sample which is representative of its amplitude.
Attempts have been made to compress PCM audio, but these attempts have generally required the addition of specialized compression/decompression equipment to existing computer equipment. The equipment typically receives PCM audio signals from an audio system, compresses the signals, and passes the compressed signals, as data, to a computer, which in turn stores the data. In order to regenerate the signals any system which retrieves the compressed data must also possess the specialized compression/decompression equipment.
The additional specialized compression/decompression equipment increases the cost and complexity of any system which employs it. Additionally, because the compression/decompression equipment generally compresses the PCM audio signal in a unique way, only other systems with compatible specialized equipment can utilize the compressed signals.
Some compression/decompression systems do not require specialized compression/decompression equipment. However, these systems do not provide real-time, lossless compression and decompression of high-quality PCM audio. They may require that PCM signals be stored and compressed off-line; a one-second audio signal would require more than one second to process. They may also provide lossy compression in order to obtain real-time operation. Lossy compression simply means that some of the signal's data is discarded in order to reduce the number of digits required to represent each sample.
It is therefore an object of the invention to compress and decompress digitized audio signals for computer system storage in a way that eliminates the need for specialized compression/decompression equipment.
It is a further object of the invention to compress and decompress digitized audio signals for computer systems with sufficient efficiency to permit real-time, lossless compression and decompression of high-quality digitized audio signals.